The field of this invention is that of deepwater control systems for the purpose of providing a supply of pressurized working fluid for the control and operation of equipment. The equipment is typically subsea or deepwater blowout preventers (BOP) which are used to shut off the well bore to secure an oil or gas well from accidental discharges to the environment, gate valves for the control of flow of oil or gas to the surface or to other subsea locations, hydraulically actuated connectors and similar devices. The fluid to be pressurized is typically an oil based product or a water based product with added lubricity and corrosion protection.
The working fluid for such control systems typically comes from accumulators. Currently accumulators have historically come in three styles which operate on a common principle. The principle is to precharge them with pressurized gas to a pressure at or slightly below the anticipated minimum pressure required to operate equipment. Fluid can be added to the accumulator, increasing the pressure of the pressurized gas and the fluid. The fluid introduced into the accumulator is therefore stored at a pressure at least as high as the precharge pressure and is available for doing hydraulic work.
The accumulator styles are bladder type having a balloon type bladder to separate the gas from the fluid, the piston type having a piston sliding up and down a seal bore to separate the fluid from the gas, and a float type with a float providing a partial separation of the fluid from the gas and for closing a valve when the float approaches the bottom to prevent the escape of gas.
Accumulators providing typical 3000 p.s.i. working fluid to surface equipment can be of a 5000 p.s.i. working pressure and contain fluid which raises the precharge pressure from 3000 p.s.i. to 5000 p.s.i.
As accumulators are used in deeper water, the efficiency of conventional accumulators is decreased. In 1000 feet of seawater the ambient pressure is approximately 465 p.s.i. For an accumulator to provide a 3000 p.s.i. differential at 1000 ft. depth, it must actually be precharged to 3000 p.s.i. plus 465 p.s.i. or 3465 p.s.i.
At slightly over 4000 ft. water depth, the ambient pressure is almost 2000 p.s.i., so the precharge would be required to be 3000 p.s.i. plus 2000 p.s.i. or 5000 p.s.i. This would mean that the precharge would equal the working pressure of the accumulator. Any fluid introduced for storage would cause the pressure to exceed the working pressure, so the accumulator would be non-functional.
Another factor which makes the deepwater use of conventional accumulators impractical is the fact that the ambient temperature decreases to approximately 35 degrees F. If an accumulator is precharged to 5000 p.s.i. at a surface temperature of 80 degrees F., approximately 416 p.s.i. precharge will be lost simply because the temperature was reduced to 35 degrees F. Additionally, the rapid discharge of fluids from accumulators and the associated rapid expansion of the pressurizing gas causes a natural cooling of the gas. If an accumulator is quickly reduced in pressure from 5000 p.s.i. to 3000 p.s.i. without chance for heat to come into the accumulator (adiabatic), the pressure would actually drop to 2012 p.s.i.
A more recent solution to this problem has been what is referred to as constant differential accumulators as is illustrated in U.S. Pat. No. 6,202,753. These accumulators use a double piston looking like a barbell which acts as mechanical summing relay. On the top side of the top piston is the gas charge similar to the more conventional accumulators. On the lower side of the upper piston is the pressurized working fluid. The lower piston is connected to the upper piston by a connecting rod. Seawater pressure is vented onto the top side of the lower piston, pushing it down and therefore pulling the upper piston down harder onto the working fluid. A vacuum is on the lower side of the lower piston and so offers no support. The net effect is that the working fluid pressure is generally equal to the sum of the nitrogen pressure plus the seawater pressure. In other words its pressure is always higher than the ambient pressure by the amount of the nitrogen pressure. This provides a good solution irrespective of depths, but provides a relatively costly construction.
During normal operating conditions the top piston will have a higher fluid pressure below the piston than the gas pressure above the piston as the rod diameter effectively reduces the piston area on the bottom side. When the top piston travels down and impacts the central bulkhead between the pistons, the pressure of the fluid below the upper piston is reduced to the environmental pressure, or less than the gas pressure above the piston. For this reason, if single direction seals are used, two seals must be used with one in each direction. When the two seals are utilized, a higher or lower pressure can be locked between the seals increasing the differential pressure across the seals. Friction loading is increased by the fact that 2 seals are used, and that the pressure differential is increased across the seals.
Single directional seals are characteristically more rugged than bi-directional seals as they are not required to do as many tasks. This advantage is frequently offset by the problem of extra sliding friction and the tendency to trap or exclude pressure from between the seals. Over the years various solutions have been attempted to alleviate this problem, but all observed have retained the double friction of the two seals. In all cases this double friction reduces the efficiency of the systems using the accumulators or pistons.